Process for preparing p-benzoquinone diketals

Abstract

P-Benzoquinone diketals of the formula ##STR1## wherein R is H, an alkyl group having from 1 to 4 carbon atoms or a halogen atom, preferably H, CH 3 , Cl or F, are prepared by anodic oxidation of benzene or alkoxybenzenes of the formula ##STR2## wherein R has the aforesaid meaning and R 1 is an alkyl radical having of from 1 to 4 carbon atoms In CH 3 OH containing A. less than about 5% by weight of H 2 O as well as B. about 0.2 to 15% by weight, calculated on the electrolyte, of at least a conducting salt selected from the group of ammonium and alkali fluorides, perchlorates, nitrates, tetrafluoroborates, hexafluorosilicates, hexafluorophosphates, benzene sulfonates, p-toluene sulfonates, quartenary ammonium and phosphonium fluorides as well as methylsulfates and additionally C. optionally about 0.5 to 10% by weight, calculated on the electrolyte, of a base to be oxidized with difficulty, preferably 2,6-lutidine, At a known anode made of a conventinal material, at a temperature of from about -20° to +60° C. When using as starting material an alkoxybenzene wherein the group OR 1 contains an alkyl radical having of from 2 to 4 carbon atoms, the resultant p-benzoquinone tetramethyl diketal contains a small quantity of the corresponding trimethyl alkyl diketal having an alkyl group of from 2 to 4 carbon atoms in the alkoxy group. The diketals obtained may be converted into the corresponding p-quinones by acidic hydrolysis and are, consequently, valuable intermediates for the preparation of the latter as well as for the transformation into the corresponding hydroquinones, which in their turn may be used in known manner in photography, as stabilizers for monomers, as starting material for dyestuff preparation etc.

What is claimed is: 
     
       1. A process for electrochemically preparing p-benzoquinone ketals which comprises anodically oxidizing benzene per se or an alkoxybenzene of the formula ##STR12## wherein R is hydrogen, an alkyl group having from 1 to 4 carbon atoms or halogen and R 1  is an alkyl group having from 1 to 4 carbon atoms, in an electrolyte which is a methanolic solution containing no more than about 5% by weight of water and about 0.2 to 15% by weight, calculated on the electrolyte, of a conductivity improving salt selected from ammonium and alkali metal fluorides, perchlorates, nitrates, tetrafluoroborates, hexafluorosilicates, hexafluorophosphates, benzene sulfonates, p-toluene sulfonates and quaternary ammonium and phosphonium salts of the formula   A.sub.x B.sub.4-x Z.sup.+ Y.sup.-     wherein A represents identical or different alkyl radicals,   B represents C 6  H 5 ,   x represents an integer of from 1 to 4,   Z represents N or P and   Y represents F or SO 4  CH 3   as well as mixtures of such salts at a pH of more than 7, at an anode of a material selected from graphite, metals of the platinum group, alloys thereof and PbO 2 , at a temperature of from about -20° to +60° C., to obtain a p-benzoquinone tetramethyl ketal of the formula ##STR13## wherein R and R 1  have the meaning given above.     
     
     
       2. A process as claimed in claim 1 wherein R represents only alkyl groups having from 1 to 4 carbon atoms, if R 1  is CH 3 .   
     
     
       3. A process as claimed in claim 1 wherein R is selected from H, CH 3 , Cl and F. 
     
     
       4. A process as claimed in claim 1 wherein the conductivity improving salt is selected from NaClO 4 , KF, (CH 3 ) 4  NF, (CH 3 ) 4  NSO 4  CH 3  and (CH 3 ) 4  PSO 4  CH 3 . 
     
     
       5. A process as claimed in claim 1 wherein the anodic oxidation is carried out at a pH of 7 to about 14. 
     
     
       6. A process as claimed in claim 1 which comprises carrying out the anodic oxidation at a temperature of from about 0° to +40° C. 
     
     
       7. A process as claimed in claim 1 which comprises carrying out the anodic oxidation at a temperature of from about +20° to +35° C. 
     
     
       8. An electrolyte adapted to be used in carrying out organic-electrochemical oxidation, said electrolyte consisting essentially of a 0.2 to 10% by weight solution of KF in methanol having a water content of no more than about 5% by weight. 
     
     
       9. A process according to claim 1 wherein the electrolyte contains from 0.5 to 10% by weight of a difficultly oxidizable base to increase the pH thereof. 
     
     
       10. A process as claimed in claim 9 wherein the difficulty oxidizable base is 2,6-lutidine. 
     
     
       11. A process for electrochemically preparing p-benzoquinone ketals which comprises anodically oxidizing benzene per se or an alkoxybenzene of the formula ##STR14## wherein R is hydrogen, an alkyl group having from 1 to 4 carbon atoms or halogen and R 1  is an alkyl group having from 1 to 4 carbon atoms, in an electrolyte which is a methanolic solution containing no more than about 5% by weight of water and about 0.2 to 15% by weight, calculated on the electrolyte, of a conductivity-improving salt selected from ammonium and alkali metal fluorides, perchlorates, nitrates, tetrafluoroborates, hexafluorosilicates, hexafluorophosphates, benzene sulfonates, p-toluene sulfonates and quaternary ammonium and phosphinium salts of the formula   A.sub.x B.sub.4-x Z.sup.+ Y.sup.-     wherein A represents identical or different alkyl radicals,   B represents C 6  H 5 ,   x represents an integer of from 1 to 4.   Z represents N or P and   Y represents F or SO 4  CH 3   as well as mixtures of such salts at a pH of more than 7, at a temperature of from about -20° to +60° C., to obtain a p-benzoquinone tetramethyl ketal of the formula ##STR15## wherein R and R 1  have the meaning given above.     
     
     
       12. A process according to claim 11 wherein the electrolyte contains from 0.5 to 10% by weight of a difficultly oxidizable base to increase the pH of the electrolyte.